You probably should specify the existing datastore(s) in the pool as well.
Check out the help:
get-help Update-AutomaticLinkedClonePool -Detailed
And it will will tell you:
You must specify at least one datastore for each usage type
The datastores that you specify override the previous setting. You must specify the existing datastores to retain them for use in this pool
This certainly does explain the error on the OS disks: datastore specs do not include any datastores configured for OS disks.
In the code you pasted it is true that there are no OS disks specified and it looks like you also did not specify the existing disk(s) in the pool.
I found a nice example in the docs: Creating and Updating Linked-Clone Desktop Pools
The example shows how to use [conservative,os,data]
Hopefully this helps to solve your issue !
Thanks, I made the script..